Sea floor records ancient Earth

By Jonathan Fildes for BBC News

Friday, 23 March 2007

A sliver of four-billion-year-old sea floor has offered a glimpse into the inner workings of an adolescent Earth. The baked and twisted rocks, now part of Greenland, show the earliest evidence of plate tectonics, colossal movements of the planet's outer shell. Until now, researchers were unable to say when the process, which explains how oceans and continents form, began. The unique find, described in the journal Science, shows the movements started soon after the planet formed. "Since the plate tectonic paradigm is the framework in which we interpret all modern-day geology, it is important to know how far back in time it operated," said Professor Minik Rosing of the University of Copenhagen and one of the authors of the paper.

Professor John Valley, a geologist at the University of Wisconsin, Madison described the work as "significant" and "exciting". "If these observations are substantiated it will be a significant line of new evidence indicating that plate tectonics was active and familiar as early as 3.8 billion years ago," he said. "That really is an important conclusion."

Plate tectonics is a geological theory used to explain the observed large-scale motions of the Earth's surface. The relatively thin outer shell of the planet is composed of two layers: the lithosphere and the asthenosphere. The lithosphere - made up of the outer crust and the top-most layer of the underlying mantle - is broken up into huge plates; seven major plates and several smaller ones. These float above the asthenosphere and move in relation to one another. Today, oceanic crust is created at plate boundaries known as mid-ocean ridges, where magma rises from the asthenospehere through cracks in the ocean floor, cools and spreads away.

As it moves away from the spreading centre towards the edges of the oceans it becomes cooler, denser and eventually starts to sink back into the mantle to be recycled. "Sea floor is not normally preserved for more than 200 million years," said Professor Rosing. Most is destroyed at subduction zones, such as those found along the edge of the Pacific Ocean, where oceanic crust plunges under the buoyant and long-lived continental crust. However, in certain circumstances, fragments of the sea floor known as ophiloites are preserved when they are scraped on to the land. This exceptional process typically occurs when continental crust begins to be sucked into a subduction zone, clogging the system. "It goes down into the subduction zone until the buoyancy of the continent arrests the process of subduction," explained Eldridge Moores, emeritus professor of geology at the University of California, Davis.

"The continent then pops back up, preserving a little bit of the overriding wedge of oceanic crust and mantle that was on the overriding plate." Ophiolites are found today in Cyprus and Oman and show a distinctive structure. At their base, crystalline rocks preserve the top layer of the mantle. Above, "fossilised" magma chambers give way to a layer of stacked vertical pipes, known as sheeted dykes. These represent the conduits through which magma is extruded onto the sea floor as pillow lavas, bulbous lobes of basaltic rock that form when lava cools quickly in contact with water. The rocks analysed in Greenland are found in an area known as the Isua Belt, a zone of intensely deformed rocks in the southwest of the island that geologists have pored over for decades. The ophiolite structure was mapped between outcrops covering 4-5km (2.5-3 miles) and shows the correct sequence of layers found in an ophiolite, except the lowest mantle portion. "You can actually recognise features that formed in a couple of minutes, 3.8 billion years ago - a quarter of all time - and you can actually go and touch them with your hand," said Professor Rosing.

Crucially, they show well preserved sheeted dykes and pillow lavas, clear evidence to many that these are the ancient remains of sea floor created by processes seen today. "What this tells you unequivocally is that the process of sea-floor spreading that we observe today appears to be present in one of, if not the, oldest sequence of rocks on Earth," said Professor Moores. "That is a significant milestone." In particular, it pushes back the oldest known evidence of plate tectonics by at least 1.3 billion years and gives scientists clues to the processes that formed the surface of the Earth today. Although the structures and processes that led to their formation would be similar to the modern era, they would not be exactly the same.

The young Earth was much hotter than now, and as it shed heat, it put many of the tectonic processes into overdrive. "If you had plate tectonics you probably would have had more plates, moving faster, and they probably would have been thinner," said Professor Moores. The rate of recycling of oceanic crust would therefore have been even quicker than today, making the fact that the rocks in Isua are preserved at all even more extraordinary. "These fragments are extremely rare," said Professor Rosing. "It's just very exciting when you get one of these glimpses when you can look back nearly four billion years in time."

[Fascinating stuff, eh?]